Distribution systems, in particular as concerns the distribution of city gas or natural gas, are currently already and will increasingly be constructed in large part of conduits formed from tubes or pipes made of polyethylene or polyamide, polybutylene, polypropylene or polyvinyl chloride. Several techniques exist for assembling such pipes. One, for example, consists in heating the end fittings of two pipes arranged opposite one another until the thermofusible material is sufficiently fluid so that, when the two end fittings are closed together under slight pressure, they melt inside one another and form a substantially gas-tight joint having a mechanical strength that is compatible with the use of the conduits being formed.
According to another technique, a heater mat is used, which is made from a resistive wire coated with an insulating varnish. The heater mat in a rectangular shape or in the form of a sheath is then given a final shape in order to be electrically powered by an automaton that is voltage or current intensity-adjustable and resistant to short circuits.
The technique of welding by means of a heater wire or a heater mat appears to be rather promising, and this is the case as concerns both the assembly process and the good performance over time of the joint thus made.
The welding of two polymer parts via fusion obtained by means of a heater mat is based on a moderate and local heating of the area of a conduit that is to be fused, which is made of a thermofusible material, e.g., polyethylene, using an electrical heating element forming a heating resistor. This welding is carried out without any addition of hardfacing material. Supplying the conductors of the heater mat with appropriate electrical power is advantageously, but not exclusively, performed by an automaton, e.g., a welding automaton commonly used for electrofusion welding.
Putting a heater mat or a heater wire into place, when this invention is not used, occurs essentially in four steps, namely:                injection molding of an impression or preform made of polyethylene or another thermofusible polymer of small thickness, generally of the order of 0.3 mm to 0.8 mm, the impression being intended to receive a resistive wire;        insertion and holding of the resistive wire inside the impression via winding;        placement of the connectors at each end of the coiled wire; and        over-molding of the connector body onto the heater implant formed at the end of the previous step.        
Producing the implant in this form is the most delicate and most costly phase, taking into account various factors capable of interfering with the process. Such factors, for example, are a break in the wire during winding, an imperfect connection of the connectors to the wire and the presence of a relatively significant residual stress differential between the impression and the connector body.
Furthermore, experience has shown that the production of the implant suffers from numerous problems in the preparation of the tubes being welded, these problems being linked primarily to the imperfect scraping of the surfaces being assembled.
This is why the thermofusion technique using a heater mat eliminates the conventional coiling of the connectors and, at the interface, results in temperature characteristics superior to those of the winding systems. In addition, fusion using a heater mat can be applied in those cases where polymer parts of various and complex geometries are to be welded together, which would not necessarily be possible with the winding system.
However, as promising and advantageous as fusion welding obtained using a heater mat might appear to be, the fact remains that it has not been possible to resolve certain difficulties inherent in the condition of the parts being welded. These difficulties are primarily due to imperfect preparation (scraping, cleaning, degreasing, . . . ) of the parts being welded, but also to a relatively advanced state of degradation (oxidation, carbonization), or else to defects of a geometric nature or significant roughness.
Other problems result from a sometimes insufficient flexibility of the mat for a given geometry of the parts, from a sometimes difficult fastening of the mat onto the part being welded and from difficulties in holding the mat in the specified position until the parts being welded have fused.
Yet another problem is that, in certain cases, it would desirable to be able to vary the supply of fusion energy from one location to another on the parts being welded. Theoretically, it would perhaps be possible to use several heater mats, each with its own power supply controlled according to the local requirements of each of the mats. However, such an approach seems very complicated to carry out at a worksite, i.e., outside of the laboratory.